Stainless steel alloy and method of forming



United States Patent G N Drawing. Application August 8, 1955 Serial N0. 527,168

4 Claims. (Cl. 75 -1 30.5)

This invention relates to a stainless steel alloy, and more particularly to an alloy having an improved yield strength in the temperature range of 600 to 1200 F. and a proper impact resistance at room temperature, which alloy is readily weldable, easily machined, and has high corrosion resistance in the above indicated high temperature range. The alloy of this invention is particularly useful in producing castings which require unusual properties as to impact strength, corrosion resistance, weldability, etc.

Certain alloys having high corrosion-resistant properties have been heretofore suggested and are designated in A.C.I. Standard Designations (Alloy Casting Institute Standard Designations and Chemical Composition Ranges for Heat and Corrosion-Resistant Castings), one of such designations being ACI CF-3M. Similar designations, showing comparable compositions, are set out in the well-recognized AISI-T318, and more particularly in A181 Type 316, A151 Type 317 and A181 Type 318. It has been suggested in some studies relating to the production of elevated temperature corrosion resistant castings that an alloy be obtained with a small percentage of ferrite in the as-cast condition, heat-treating the alloy at 2050 F. for 4 hours to place all carbides in solution, water quenching to keep them in solution, maintaining the material for 5 hours at 1550 F., and then cooling in still air to transform the small percentage of ferrite into sigma phase.

The results showed tensile values determined at 700 F.

lower than the desired 25,000 p.s.i.

In the foregoing work, it was discovered that consid- :erable difliculty Was encountered in attempting to maintain the ferrite content at 510%. Such a ferrite content was desired to give the desired weldability. The lower the ferrite content, the more difiicult it is to weld .without encountering hot shortness. On the other hand, as the ferrite content is lowered, the impact values improve. To produce a product meeting the critical requirements above outlined, it is desirable to maintain the ferrite content between 5 and 10%.

We have discovered that an alloy can be produced meeting the extreme requirements heretofore set out by employing a chemical range or composition which will be hereinafter set out, and thereafter, in the treatment of the starting materials, making tests at the furnace itself and adding the necessary alloys to the bath to bring the bath to the proper ferrite range. In this process, the chemical range of the ingredients plays an important part, as will be set out more fully hereinafter.

An object of the present invention is to provide a simple and effective process whereby a stainless steel alloy having the critical and high requirements heretofore stated may be obtained in regular plant operation. Another object is to provide a formula or chemical range which is effective in the carrying out of the process for producing the new alloy. A still further object is to provide a chemical range and method for producing an alloy having an improved yield strength in high temperature ranges, good impact resistance at room temperature,

ice

excellent corrosion resistance at high temperatures, while being readily weldable and easily machined. Other specific objects and advantages will appear as the specification proceeds. g

In one embodiment of our invention, we form a bath in a furnace such as an electric furnace, containing the following chemical ingredients:

Chromium 17.2518.25%.

Nickel 12.00-13.00%.

Carbon 0.034% max. Molybdenumnm 2.25-3.00%.

Silicon 0.350.75%.

Manganese 1.0% max.

Columbium and tantalum 10 times carbon content.

.Any suitable procedure in the combining of the ele ments in the bath may be employed. By way of example, the charge may consist of clean mild steel, nickel, and molybdenum, which may be melted down in a basiclined arc furnace until the bath reaches a temperature in the neighborhood of 3000 F. Treatment with lime and with oxygen may follow, and after the boil, ferrosilicon and ferrochromium may be added. Later lime may be added again and low carbon ferromanganese and ferro columbium-tantalum. A sample may be taken for carbon determination. To build up the silicon and chromium contents to the desired level, selected amounts of ferrochromium, ferrosilicon and ferro-chromium-silicon may be used.

In order to determine the ferrite content, a series of samples are taken and the magnetic permeability of each sample determined by a standard permeability tester. The necessary alloys are then added to the bath to bring the bath to the proper ferrite range. The method of making the ferrite test is described in greater detail in the copending application of Leroy E. Fink, Serial No. 499,765, now abandoned.

The embrittlement or cracking tendency of the alloy known as hot shortness may further be combated by adding to the metal vanadium and titanium and also, if desired, a small amount of boron. It is preferred to add the titanium in the amount of 0.01 to 0.06% and the vanadium in the proportion of about 0.02 to 1%. Very good results are obtained when the vanadium and titanium are added tin the proportions of 0.05 to 0.25% vanadium and 0.01 to 0.15% titanium. A more complete description of the addition of such metals to combat hot shortness is set forth in the copending application of Leroy E. Fink, Serial No. 485,341, now abandoned. It will be understood that the adding of the vanadium, titanium and boron is an optional procedure and may be omitted, if desired.

It is also possible to produce the desired alloy using columbium alone, and without tantalum, at eight times the carbon content, and the ranges may be set'out as follows:

We have discovered that within the ranges set out above and on column 2 lines 9-15 hereof, the desired product can be produced if the ferrite content in the final alloy is kept at 5'l0%. We have found that the lower the ferrite content, the more difiicult to weld without encountering hot shortness, while with a lowered ferrite content, the impact values improve. By maintaining the ferrite content in the range of 10%, the ranges described in the foregoing tables may be effectively employed in producing the important new alloy. Various tests may be employed in determining the ferrite content dur- The charge consisting of clean mild steel, nickel and in the ro ress of the r t' f r xam le the 5 f g of the f i g g ihs gy i f g molybdenum is melted down m a baslc lmed arc furnace mm the temperature of the bath reaches 2950-3000 F., to above, or by employlng other suitable test methods. a

after whlch l1me 15 added. In th1s example, a Lectro By utlhzmg the ranges descnbed and adding the necessary 1 in redients to maintain the ferrite content of 5-107 me 5126 furnace was employed Following thls a ghiuhl successful result is achieved the electrodes and roof are lifted and the heat is boiled 5 mducmm of th met a1 3110 is the hemical with oxygen for a predetermined time depending on e c charge Weight. After the boil, 75% ferro-silicon and composmon of the alloy i determines. the phase Simplex ferrochromium are added The power is then crystal structuie of sohd metal and m the pracnce switched to #1 tap which was set for 9000 amperes at of the present invention, we find that it is desirable to ob- 215 volts when the Simplex ferrochmme is almost i a rough control of the composmpli and character 15 melted, the balance of the lime is added and the power is Isms T alloy prqduct by cox-nbmmg the all-037mg switched to #2 tap which was set for 8800 amperes at materlals 1n predeterm ned proportions as set out in the 185 volts. Then when the lime is completely melted i? preferred chemlczil ranges modlfymg the extra low carbonderromanganese and ferro columbium: posmon as liter determined by the tests. made at i tantalum is added. A spoon sample is taken for carbon furnace to glve the desired product f the ferglte determination. (The power is turned 011 while the carbon of 5 to The added.mate1:1als are austfimte' is determined and resumed immediately after the deterormers and ferrite-formers having different grades of minafion is made) To build up the Silicon and chro Potency exampletndolybdenllm f chmmmm mium contents to the desired level, selected amounts of and columbium are ferritizers, while nickel, carbon, and fermchmmium fermsmcon and fermchmmiunksm; manganese are austenitizers. Well known formulas incon are used i i Purpose dlcate the potency 9f these elements! and the-elements When homogeneity is achieved and the heat is ready maybe added to bring about the deslred chemical comto be tapped a Sample is taken with a slagged spoon posit on as the magnetic permeabihty tests are made. In and poured to a copper mold The metal is allowed makmg pie nests at the furnac? a Sampleis wlthdmwn to cool in this mold until black It is then removed and sohdlfied-and the magnetic pfirmeablhty measured and quenched in water. When cool the bottom surface standard mstwmenlg such for examlile i perme' of the sample is tested with a Severh Engineering Comgggg igggg g fif g fif 2:? fi igizf i i $2 3 pany Permeability Tester. Depending on the results'of i this test small amounts of alloy agents (e.g. chromium the magi-16m permeaplhty of the Samp 1c is compared nickel, inanganese, silicon) may be added tr adjust the Ymh a previously estabhshed standard value Formsponm chemical composition to obtain the desired ferrite conmg to preselected proportion? of magneuc arid nontent. After any addition is made to the molten bath n-lagneuc componentsth? magneuc compol-lent bemg tierin the furnace the metal is well stirred to obtain a 1 and the fi g homo eneous b ath Another sample is then taken This ext, a meta a oying matena is a e an t s 1s rocedure 1s continued until the des1red ferrite co ten elther a minenahwhlch has the property of mcfeasmg 40 5s determined by magnetic permeability is obtzi ineci or a materlal which has the property of decreasing, the when this occurs the temperature of thebath is deter: relative proportlons of magnetlc to nonmagnetic commined and if temperature is F or greater ponents 1n the metal alloy. It 1s usually necessary to V the heat is ta ed repeat the steps a number of times until the last meas- PP I ured value for magnetic permeability closely approxi- E l [I mates the standard value, thereby indicating that the preselected proportions of magnetic and nonmagnetic com- A of examples 1S Illustrated byfhs Pg ponents have been achieved and more particularly bringtable Settmg forth f analyses d P i/ 1 properties ing the ferrite content within the selected range of 5 0f the catlngs ObtHIPBd- The castlngs ve the necesto 10%. sary physical propertles and are weldable. The analyses For the efifective operation of the process, we found demonstrate the interaction of the elements themselves that the chemical range heretofore set out provides the 9 the ferrite COnstllllfiHt chromium rough control which is necessary and efiective for pro- 18 balanced 01? y austenlte-folmlng lllckel, Carbon ducing the new alloy, and in this chemical range the manganese.

Chemical Composition, percent Mechanical Properties Impact Yield Heat Tensile Yield Eloug. BHN inft.-lbs., Strength,

No. Strength, Strength, i112, Reduc. Gharpy p.s.i. at

0 Mn Si Cr Ni Mo Ob S P p.s.i. (2 per; inareag Keyhole 7001".

0 S8 cen ercen p.s.i. p

chromium may be from 17.25 to 18.25%. The nickel may be 12.00 to 13.00%, the carbon should be 0.034% maximum, and the molybdenum 2.253.00%. The silicon should be 0.35-0.75%, and the manganese 1.0% maximum. The columbium plus tantalum should be 10 times the carbon content.

70 While, in the foregoing specification, we have set forth specific ranges and analyses and steps of procedure in considerable detail for the purpose of illustrating the invention, it will be understood that such details may be varied widely -by those skilled in the art without departing from the spirit of our invention.

We claim:

1. In a method of producing a stainless steel alloy having a pre-selected proportion of ferrite, the steps of combining with steel in a molten bath and in the proportions indicated, chromium l7.2518.25%, nickel 12.00-13.00%, carbon 0.034% maximum, molybdenum 2.25-3.00%, silicon 0.350.75%, manganese 1.0% maximum, and columbium plus tantalum times the carbon content, drawing samples from the bath and determining the ferrite content thereof, and adding to the same molten bath an alloy material selected from the group consisting of ferrite-formers and austenite-formers to bring the ferrite content between 5 and 10%.

2. The process of claim 1, in which the ferrite content is determined by determining the magnetic permeability of the Withdrawn bath sample.

3. The process of claim 1, in which a small percentage of titanium and vanadium are added, the vanadium being in the range of 0.02 to 1% and the titanium 'being in the range of 0.01 to 0.06%.

4. In a process for producing a readily-weldable stainless steel alloy of high impact resistance at room temperature, the steps of combining with steel in a molten bath, and in the proportions indicated, chromium 17.25- 18.25%, nickel IZOD-13.00%, carbon 0.034% maximum, molybdenum 2.25-3.00%, silicon 0.35-0.75%, manganese 1.0% maximum, and columbium 8 to 10 times the carbon content, periodically drawing molten samples from the bath and determining the ferrite content thereof, adding to the same molten bath alloy materials selected from the group consisting of ferrite-formers and austenite-formers to bring the ferrite content between 5 and 10%, and then pouring the molten metal.

References Cited in the file of this patent UNITED STATES PATENTS 2,159,724 Franks May 23, 1939 2,240,672 Scherer et al. May 6, 1941 2,481,386 Bloom Sept. 6, 1949 2,544,336 Linnert Mar. 6, 1951 OTHER REFERENCES Transactions, American Society for Metals, vol. 39, 1947, pages 868 to 871. Published by the A.S.M., Cleveland, Ohio.

Buck et al.: ASTM Symposium on Evaluation Tests for Stainless Steels, 1950, Special Tech. Publ. No. 93, pages 56-86. Published by the American Society for Testing Materials, Philadelphia, Pa.

Binder et al.: ASTM Symposium on Evaluation Tests for Stainless Steels, 1950, Special Tech. Publ. No. 93, pages 146-182. Published by the American Society for Testing Materials, Philadelphia, Pa. 

1. IN A METHOD OF PRODUCING A STAINLESS STEEL ALLOY HAVING A PRE-SELECTED PROPORTION OF FERRITE, THE STEPS OF COMBINING WITH STEEL IN A MOLTEN BATH AND IN THE PROPORTIONS INDICATED, CHROMIUM 17.25-18.25%, NICKEL 12.00-13.00%, CARBON 0.034% MAXIMUM, MOLYBDENUM 2.25-3.00%, SILICON 0.35-0.75%, MAGANESE 1.0% MAXIMUM, AND COLUMBIUM PLUS TANTALUM 10 TIMES THE CARBON CONTENT, DRAWING SAMPLES FROM THE BATH AND DETERMINING THE FERRITE CONTENT THEREOF, AND ADDING TO THE SAME MOLTEN BATH AN ALLOY MATERIAL SELECTED FROM THE GROUP CONSISTING OF FERRITE-FORMERS AND AUSTENITE-FORMERS TO BRING THE FERRITE CONTENT BETWEEN 5 AND 10%. 